2. Field of the Invention
This invention is related to apparatuses and methods for adjusting a material's fluid content and effective stresses and, in one embodiment tests of geologic materials (e.g. soils, sands, clays, rocks, coal, etc.) and, in one aspect, to triaxial test apparatuses and methods for low permeability rock, e.g. shales, to determine various properties of the rock such as mechanical strength properties, stress history, compression (compaction), history, compressibility properties, effective stresses, and pore pressure. In certain embodiments this invention is directed to triaxial test apparatuses and methods for multi-stage tests of low permeability rocks in which a sample is stressed almost to failure at one water content or in which a sample is consolidated; some sample fluid is then expelled from the sample; and the test is then repeated at several water contents and then at a final water content is loaded to failure.
2. Description of the Related Art
Wellbore stability has been frequently identified as one of the major sources of trouble costs for drilling in shale. In order to minimize wellbore stability related trouble costs, testing programs have been developed to build data bases and generalize the mechanical properties of different shales. Very high quality and useful results have been obtained. However, such test programs are very tedious and require many tests (e.g. 15-20) to quantify the properties for one shale type.
FIG. 1 illustrates a conventional triaxial test apparatus for rock testing. The test is performed inside an enclosed hydraulic cell and a confining fluid pumped into the cell provides a confining pressure on a rock sample specimen to be tested. The rock sample is placed on the bottom end cap for support. During the test, hydraulic pressure is applied around the sample to generate an isostatic confining stress and then an axial load (usually via a load piston) is applied to the top end cap to generate a deviatoric (shear) stress. The prior art does not teach monitoring water content during a triaxial test nor does it teach controlling and adjusting water content of low permeability materials, such as shales. The prior art does not teach a multi-stage test wherein a low permeability rock sample is stressed to a point near its shear failure, without reaching shear failure, then withdrawing some sample fluid to increase the rock's pore pressure and to then again stress the rock, all in one test.
Applicants are unaware of any prior art disclosing triaxial test apparatuses and methods for multi-stage tests of low permeability rock, such as shales, which permit step-wise reduction and/or increase of sample water content. Applicants are also unaware of any prior art disclosing test apparatuses and methods that can accurately determine the strength--mean effective stress relationship of a low permeability material within one test.
There has long been a need for triaxial test apparatus and methods for low permeability rock for a multi-stage test. There has long been a need for such apparatuses and methods in which a single test of a relatively small amount of a core sample can yield a variety of properties. There has long been a need for triaxial test apparatuses and methods which could effectively and accurately measure different pore pressures in one sample of low permeability rock in one test.